I don't know what the entire capability of a subwoofer driver is. Apparently though, it's nowhere near the upper reaches of the stated frequency response. There's a difference between measuring a driver's range as an absolute and determining where it works best. A smaller, high excursion driver is best used for low frequencies when space is a factor and this is the reason many are made this way. The other way to achieve high output at low frequencies is by using a larger diameter driver- both are effective because they move more air than a low excursion small driver or a driver that's too small for the acoustic space. By definition, a subwoofer operates in the range below that of a woofer, so....
